Bacteria pressed into service as living transistors

BUFFALO, N.Y. -- State University of New York (SUNY) researcherswho were studying the problem of bacteria that sabotage the yield on
semiconductor lines may have found a way to use them to create
"biotransistors," harnessing a particular bacterium's photosensitivity to
create an optoelectronic switching element.

Engineers will benefit not only from the deeper understanding they will
gain of how bacteria elude even the harshest clean-room procedures,
but also from the next-generation preview of living transistors inside
the biochips of the future.

The researchers recently discovered that errant bacteria survive in
the cleanest of clean rooms by inducing the top semiconductor layer
on chips to grow over them, thereby embedding themselves inside the
chip.

When SUNY researcher Robert Baier started to investigate the role
bacteria play in the yield problem, he knew they were avoiding even
the most stringent attempts to eradicate them -- the bugs just
wouldn't die. How could bacteria survive where no other living thing
can? Baier, funded by the National Science Foundation, found the
answer, but it wasn't what he expected.

"When we started this study, we were just trying to find the source
of bacteria in the fab, and how they could remain alive after all the
heroic measures to eradicate them with ultraviolet light, ozone and
everything else including a dollar a gallon to purify the water," said
Baier, who is director of the Center for Biosurfaces at SUNY.

Other scientists and engineers participated in the research at the
Center for Microcontamination Control at the University of Arizona,
the Rensselaer Polytechnic Institute in New York and the Center for
Environmentally Benign Semiconductor Manufacturing at the University
of Arizona.

The problem wasn't people in the fab getting sick -- those kinds of
bacteria were easy to kill. Rather, it concerned some clever bugs that
just wouldn't die, no matter what -- bacteria that can survive in the
vacuum of space, or inside a volcanic vent at the bottom of the sea.
They can hibernate indefinitely and only need the slightest bit of light
to wake up and thrive anew.

"We found that these extremely hard-to-kill bacteria were coming in
with the ultrapure water, and the way they survived our calculated
assault was to capture a tiny bit of semiconductor that had dissolved
in the ultrapure water and start growing," said Baier.

Once the bacterium sticks a molecule of semiconductor to itself, other
dissolved crystals spontaneously attach themselves to the formation,
growing islands atop silicon wafers during a subsequent vapor
deposition step.

In short order, the bacteria have encased themselves inside armored
shells of semiconductor, making them impervious to all the attempts
by clean-room personnel to kill them. "These bacteria can cause a lot
of problems in the clean room, like shorting out adjacent lines on
chips, and inside these armored shells they are almost impossible to
kill," said Baier. "Now we are turning a problem into a feature. A plant
is basically a single-electron photonic device converting light into
electricity. If we embed a photosensitive bacteria inside a chip, we
have the beginnings of a biotransistor."

Baier's goal of harnessing bacteria as the active element in a
transistor may not be as far-fetched as it sounds -- at least his
theory sounds convincing. He points out that copper is a conductor
because it has one free electron per atom to contribute to current
flow. Semiconductors are called "semi" because they have only about
one free electron per thousand atoms, depending on doping levels.
Current flow in those semiconductors, unlike copper, can be precisely
controlled by parameters that match the parameters of bacteria,
according to Baier, enabling regular transistors to switch from an
insulator into a conductor by changing state.

'Many uses'

These small charge transfers, Baier contends, are just what happens
in common biological processes like respiration and photosynthesis. In
fact, he believes that the current flowing in a semiconductor can be
controlled by the chlorophyll in a single cell. For instance, when light
shines on a photosensitive bacterium, it yields up an electron that
could be used to switch a primitive biotransistor. "This is a new class
of biochips, which I believe can be adapted to many uses, but at
present it's at a primitive stage, like the crude crystal detectors that
preceded today's radios," said Baier.

The theory is that doping semiconductors is always done to disrupt
the perfect lattice, making free electrons or "holes" available in "n-"
and "p-" type semiconductors, respectively. Likewise, if a biological
atom, say phosphorus with five electrons from a bacterium's cell, is
doped into a silicon crystal that only needs four, it then makes the
fifth electron available, enabling biological cells to serve as metabolic
"sources" and "sinks" for electrons and "holes" in biotransistors.

"Biological membranes have been known to develop potentials of a
million volts per centimeter. They are perfect for semiconductors, and
biochips using them could be made as small as five microns on a side,"
said Baier.

According to Baier, most of the steps used to build chips today will
continue to be used in manufacturing his biochips. For instance,
masks will be used on the semiconductor doped with bacteria, as will
diffusion, sputtering and other common deposition techniques. All the
other common devices like resistors and capacitors will be built into
circuits connected to the biotransistors.

Optical amps

Biotransistors will also simplify optical communications by amplifying
optical beams the way a normal transistor amplifies electrical current.
And Baier envisions the construction of light-sensitive heterojunctions
where lattices of different energy gaps are "biodoped" so that their
crystalline lattices mesh imperfectly, creating atomic-scale defects
and strains with useful photonic and electrical side effects to drive
circuitry. Such heterojunctions would harness complicated biophysical
reactions to provide a tunable variable for design problems.

For now, Baier will be satisfied if he can build a "crystal" radio with his
biochip architecture. His approach will be to follow the recipe of
Bardeen and Brattain when they invented the world's first transistor.
As with the original, Baier plans to attach two fine wires only
micrometers apart, but this time in a biodoped germanium crystal that
has been bonded to a metal disk. The assembly will be housed inside a
metal cylinder electrically grounded to the disk. Bardeen and Brattain's
"cat's whisker" will be connected to a meter to register success or
failure.